O presente trabalho versa sobre a modelagem de uma chama turbulenta difusiva usando a técnica de simulação de grandes estruturas turbulentas (LES), juntamente com o modelo termo-químico de folha de chama (flame sheet model) e o conceito de fração de mistura como escalar conservativo. Este trabalho também é pioneiro de utilização de LES com reação química no Brasil, podendo colaborar para o desenvolvimento desta técnica na área de combustão. O trabalho consiste na construção e validação das rotinas computacionais de um código CFD, baseado em LES e com flexibilidade para uma futura utilização de cinética química detalhada de combustão (EDC/ISAT), para casos complexos onde modelos mais simples, como a fração de mistura, são falhos. O programa será validado em uma chama de difusão turbulenta não-confinada de metano (CH4), para a qual existem dados experimentais na literatura [61,62] e utilizados pela comunidade acadêmica em excelência (Stanford, TU-Darmstadt, Imperial College, Cornell University etc). As características da implementação numérica do código permitirão sua expansão futura para outras aplicações em: queima de combustíveis líquidos, combustão em câmaras fechadas e fornalhas com a inclusão de modelo de radiação. / The present work is about modeling a diffusive turbulent flame using the Large-Eddy Simulation approach (LES) and the Flame Sheet model as the chemical model with the mixture fraction concept as the conservative scalar. This work is pioneer in the sense of using LES and reactive flow in Brazil, making possible the development for LES techniques in the combustion area. The work is intended to construct and validate a CFD code based on LES and with future flexibility for a more detailed combustion chemical model (EDC/ISAT) for complex flows, where simple models are failed, like the mixture fraction. The program will be validated for a turbulent diffusion methane (CH4) flame not confined, which there are some experimental data on the specialized literature [61,62], and commonly used by the academic community (Stanford, TU-Darmstadt, Imperial College, Cornell University etc). The features of the numerical code implementation will make possible future expansion of its use in other applications: liquid fuel burning, combustion chambers and ovens with the radiation model inclusion.

Escoamentos turbulentos têm sido por muitos anos o objetivo de importantes estudos para descobrir sua dinâmica. Dentre as características mais significativas, destaca-se a multiplicidade de escalas que os caracterizam, desde as maiores estruturas (baixas freqüências) controladas pela geometria que as geram, até as menores estruturas (altas freqüências) limitadas pela viscosidade do fluido. Uma idéia importante é o conceito de vórtices que está ligado a melhorias nas técnicas de visualização, tanto em laboratório quanto em experimentos numéricos. Estes vórtices têm um importante papel em numerosas aplicações tecnológicas, sendo necessário entender a dinâmica da organização de seus movimentos para controlar mecanicamente sua produção ou supressão. Neste contexto, a análise de um misturador de ar em um escoamento em tê é o principal objetivo de estudo deste trabalho. A geometria em tê é bastante simples, mas propicia o aparecimento de um escoamento com passagem de vórtices. Testes experimentais do escoamento, com duas entradas de ar com temperaturas diferentes, foram realizados no Laboratório de Engenharia Térmica e Fluidos da Escola de Engenharia de São Carlos da Universidade de São Paulo (LETeF - EESC - USP). As medidas de temperatura foram obtidas com termopares instalados ao longo da tubulação. Com o software CFX foram realizados estudos com métodos numéricos LES e DES aplicados ao escoamento. Estes resultados computacionais foram comparados com os dados experimentais, através da análise tempo-freqüência. Estudos preliminares do escoamento mostram regiões com passagem de vórtices, e a habilidade da técnica de análise tempo-freqüência em caracterizar a existência e a forma destas estruturas turbulentas. / Turbulent flows have been the objective of important studies to discover its dynamics. One important characteristic of these flows is the multiplicity of scales, since the large structures (low frequencies) controlled by the geometry that generates them, until the small structures (high frequencies) limited by the fluid viscosity. An important idea is the concept of vortices that it is associated with the improvements in the visualization techniques, in laboratory or numerical experiments. These vortices have an important function in many technological applications. In each one of these fields, it is necessary to understand the dynamics of its movements to control the mechanisms for producing or suppressing these vortices. In this context, the analysis of an air mixing in a tee junction is the main objective of this work. The tee geometry is sufficiently simple, but contributes for the appearance of a flow with vortices transition. Experimental tests with two different air temperatures inlets were done at the Thermal and Fluid Engineering Laboratory of the University of São Paulo at São Carlos (LETeF - EESC - USP). The measures of temperature were acquired with thermocouples installed along the pipe. Numerical studies with LES and DES methods using CFX software were applied to the flow. These computational results were compared with the experimental data through the time-frequency analysis. Preliminary studies of the flow show vortices transition regions and the ability of time-frequency technique in describing the existence and shape of turbulent structures.

Avec l’accroissement considérable de la puissance de calcul, les simulations aux grandes échelles (SGE) sont maintenant utilisées de façon routinière dans de nombreuses applications d’ingénierie. Les modèles de combustion usuels utilisés dans les SGE sont le plus souvent basés sur une hypothèse d’équilibre entre le mouvement des structures turbulentes et le plissement de la surface de la flamme. Ils s’écrivent alors sous forme d’expressions algébriques fonctions de grandeurs connues aux échelles résolues ainsi que de paramètres dont l’ajustement est à la charge de l’utilisateur selon la configuration étudiée et les conditions opératoires. Le modèle dynamique récemment développé ajuste automatiquement au cours du calcul les paramètres de modélisation qui peuvent alors dépendre du temps et de l’espace. Cette thèse présente une étude détaillée d’un modèle dynamique pour la simulation aux grandes échelles de la combustion turbulente prémélangée. L’objectif est de caractériser, explorer les avantages et les inconvénients, appliquer et valider le modèle dynamique dans plusieurs configurations. / Large eddy simulation (LES) is currently applied in a wide range of engineering applications. Classical LES combustion models are based on algebraic expressions and assume equilibrium between turbulence and flame wrinkling which is generally not verified in many circumstances as the flame is laminar at early stages and progressively wrinkled by turbulent motions. In practice, this conceptual drawback has a strong consequence: every computation needs its own set of constants, i.e. any small change in the operating conditions or in the geometry requires an adjustment of model parameters. The dynamic model recently developed adjust automatically the flame wrinkling factor from the knowledge of resolved scales. Widely used to describe the unresolved turbulent transport, the dynamic approach remains underexplored in combustion despite its interesting potential. This thesis presents a detailed study of a dynamic wrinkling factor model for large eddy simulation of turbulent premixed combustion. The goal of this thesis is to characterize, unveil pros and cons, apply and validate the dynamic modeling in different flow configurations.

L'assimilation de données en tant qu'outil pour la mécanique des fluides a connu une croissance exponentielle au cours des dernières décennies. La possibilité de combiner des mesures précises mais partielles avec un modèle dynamique complet est précieuse et a de nombreuses applications dans des domaines allant de l'aérodynamique, à la géophysique et à l’aéraulique. Cependant, son utilité reste limitée en raison des contraintes imposées par l'assimilation de données notamment en termes de puissance de calcul, de besoins en mémoire et en informations préalables. Cette thèse tente de remédier aux différentes limites de la procédure d'assimilation pour faciliter plus largement son utilisation en mécanique des fluides. Un obstacle majeur à l'assimilation des données est un coût de calcul prohibitif pour les écoulements complexes. Une modélisation de la turbulence à grande échelle est intégrée à la procédure d'assimilation afin de réduire considérablement la coût de calcul et le temps requis. La nécessité d'une information volumétrique préalable pour l'assimilation est abordée à l'aide d'une nouvelle méthodologie de reconstruction développée et évaluée dans cette thèse. L'algorithme d'optimisation reconstruit les champs 3D à partir d'observations dans deux plans orthogonaux en exploitant l'homogénéité directionnelle. La méthode et ses variantes fonctionnent bien avec des ensembles de données synthétiques et expérimentaux fournissant des reconstructions précises. La méthodologie de reconstruction permet également d’estimer la matrice de covariance d’ébauche indispensable à un algorithme d’assimilation efficace. Tous les ingrédients sont combinés pour effectuer avec succès l'assimilation de données variationnelles d'un écoulement turbulent dans le sillage d'un cylindre à un nombre de Reynolds transitoire. L'algorithme d'assimilation est validé pour des observations volumétriques synthétiques et est évalué sur des observations expérimentales dans deux plans orthogonaux. / Data assimilation as a tool for fluid mechanics has grown exponentially over the last few decades. The ability to combine accurate but partial measurements with a complete dynamical model is invaluable and has numerous applications to fields ranging from aerodynamics, geophysics, and internal ventilation. However, its utility remains limited due to the restrictive requirements for performing data assimilation in the form of computing power, memory, and prior information. This thesis attempts at redressing various limitations of the assimilation procedure in order to facilitate its wider use in fluid mechanics. A major roadblock for data assimilation is the computational cost which is restrictive for all but the simplest of flows. Following along the lines of Joseph Smagorinsky, turbulence modelling through large-eddy simulation is incorporated in to the assimilation procedure to significantly reduce computing power and time required. The requirement for prior volumetric information for assimilation is tackled using a novel reconstruction methodology developed and assessed in this thesis. The snapshot optimisation algorithm reconstructs 3D fields from 2D cross- planar observations by exploiting directional homogeneity. The method and its variants work well with synthetic and experimental data-sets providing accurate reconstructions. The reconstruction methodology also provides the means to estimate the background covariance matrix which is essential for an efficient assimilation algorithm. All the ingredients are combined to perform variational data assimilation of a turbulent wake flow around a cylinder successfully at a transitional Reynolds number. The assimilation algorithm is validated with synthetic volumetric observation and assessed on 2D cross-planar observations emulating experimental data.

The launch of large stratospheric balloons is highly dependant on the meteorological conditions at ground level, including wind speed. The balloon launch base Esrange Space Center in northern Sweden currently uses forecasts delivered through the Swedish Meteorological and Hydrological Institute to predict opportunities for balloon launches. However the staff at Esrange Space Center experience that the current forecasts are not accurate enough. For that reason the Weather Research and Forecasting model is used to improve the forecast. The model performs a Large Eddy Simulation over the area closest to Esrange Space Center to predict wind speed and turbulence. During twelve hypothetical launch days the improved forecast have an overall accuracy of 93% compared to the old forecast accuracy of 69%. With some improvements and the right computational power the system is thought to be operationally viable.

The increasing use of Intelligent Transport System (ITS) can enable very close vehicle spacings which generally results in a net drag reduction for the resulting convoys. The majority of vehicle development has, to date, been for vehicles in isolation, thus the study of interaction effects is becoming increasingly important. The main objective of this research is to investigate the use of Computational Fluid Dynamics (CFD) for understanding convoy aerodynamics and to further the understanding of airflow interaction between vehicles via CFD. In this study, time-averaged characteristics of a simplified, generic passenger vehicle, called the Ahmed car model, after Ahmed et.al (1984) is investigated computationally using the available commercial CFD code, Fluent version 6.1.22. Three different platoon combinations were analysed for the current study which includes a two, three and six model platoons for various rear end configurations of the Ahmed model geometry. Experiments were conducted in RMIT University Industrial Wind Tunnel for analysing the effects of drafting on drag coefficients using two different scales of Ahmed car models. This is an extension to the previous study performed on two 100% scales of Ahmed models (Vino and Watkins, 2004) and the results for both the current and previous experiments were compared using CFD. The CFD proved to be a useful technique since its results compared reasonably well for both the current and the previous experiments on drafting, using Ahmed models of identical (30°) rear slant configurations. However, near critical rear slant angles (~30°) for isolated vehicles some discrepancies were noted. The reasonable validation of experimental results enabled the study to be extended further computationally using CFD, to analyse the effects of inter-vehicle spacing on a platoon of 3 and 6 models for various rear end configurations (between 0° and 40°), in an attempt to provide useful information on vehicle-wake interaction for the Future Generation Intelligent Transport System (FGITS). Critical gaps were identified via CFD for the case of a two, three and six model platoons and the simulations clearly exposed the reasons for these critical gaps. At extremely close proximity, the models experienced more pressure recovery at their rear vertical base, which reduced the drag coefficient. Surprisingly, at some of the close vehicle spacings, the drag coefficients reached values that were higher than that of a vehicle in isolation. This was found due to the high momentum flow impingement to the fore body of the model and was similar to results found in physical experiments. Thus the current CFD analysis revealed that rear slant angle of the model and the inter-vehicle spacing greatly influences the wake structures and ultimately the vehicles aerodynamic drag coefficients in platoons. Even though the current CFD model (Realizable k-B turbulence model) predicted the basic flow structures such as the C-pillar vortices from the rear slant and 2D horse shoe vortices in the model's vertical rear base, the separation bubble on the rear slant that supplies energy to the strong C-pillar vortices was not replicated accurately, which is evidenced from the flow structure analysis. Hence it is recommended for further work, that the study should be extended using the Reynold's stress models or the Large Eddy Simulation (LES) turbulence models for flow structure observation and analysing vortex interactions between the models.

The objective of this research is to develop an efficient and accurate methodology to resolve flow non-linearity of fluid-structural interaction. To achieve this purpose, a numerical strategy to apply the detached-eddy simulation (DES) with a fully coupled fluid-structural interaction model is established for the first time. The following novel numerical algorithms are also created: a general sub-domain boundary mapping procedure for parallel computation to reduce wall clock simulation time, an efficient and low diffusion E-CUSP (LDE) scheme used as a Riemann solver to resolve discontinuities with minimal numerical dissipation, and an implicit high order accuracy weighted essentially non-oscillatory (WENO) scheme to capture shock waves. The Detached-Eddy Simulation is based on the model proposed by Spalart in 1997. Near solid walls within wall boundary layers, the Reynolds averaged Navier-Stokes (RANS) equations are solved. Outside of the wall boundary layers, the 3D filtered compressible Navier-Stokes equations are solved based on large eddy simulation(LES). The Spalart-Allmaras one equation turbulence model is solved to provide the Reynolds stresses in the RANS region and the subgrid scale stresses in the LES region. An improved 5th order finite differencing weighted essentially non-oscillatory (WENO) scheme with an optimized epsilon value is employed for the inviscid fluxes. The new LDE scheme used with the WENO scheme is able to capture crisp shock profiles and exact contact surfaces. A set of fully conservative 4th order finite central differencing schemes are used for the viscous terms. The 3D Navier-Stokes equations are discretized based on a conservative finite differencing scheme, which is implemented by shifting the solution points half grid interval in each direction on the computational domain. The solution points are hence located in the center of the grid cells in the computational domain (not physical domain). This makes it possible to use the same code structure as a 2nd order finite volume method. A finite differencing high order WENO scheme is used since a finite differencing WENO scheme is much more efficient than a finite volume WENO scheme. The unfactored line Gauss-Seidel relaxation iteration is employed for time marching. For the time accurate unsteady simulation, the temporal terms are discretized using the 2nd order accuracy backward differencing. A pseudo temporal term is introduced for the unsteady calculation following Jameson's method. Within each physical time step, the solution is iterated until converged based on pseudo time step. A general sub-domain boundary mapping procedure is developed for arbitrary topology multi-block structured grids with grid points matched on sub-domain boundaries. The interface of two adjacent blocks is uniquely defined according to each local mesh index system (MIS) which is specified independently. A pack/unpack procedure based on the definition of the interface is developed to exchange the data in a 1D array to minimize data communication. A secure send/receive procedure is employed to remove the possibility of blocked communication and achieve optimum parallel computation efficiency. Two terms, "Order" and "Orientation", are introduced as the logics defining the relationship of adjacent blocks. The domain partitioning treatment of the implicit matrices is to simply discard the corner matrices so that the implicit Gauss-Seidel iteration can be implemented within each subdomain. This general sub-domain boundary mapping procedure is demonstrated to have high scalability. Extensive numerical experiments are conducted to test the performance of the numerical algorithms. The LDE scheme is compared with the Roe scheme for their behavior with RANS simulation. Both the LDE and the Roe scheme can use high CFL numbers and achieve high convergence rates for the algebraic Baldwin-Lomax turbulence model. For the Spalart-Allmaras one equation turbulence model, the extra equation changes the Jacobian of the Roe scheme and weakens the diagonal dominance. It reduces the maximum CFL number permitted by the Roe scheme and hence decreases the convergence rate. The LDE scheme is only slightly affected by the extra equation and maintains high CFL number and convergence rate. The high stability and convergence rate using the Spalart-Allmaras one equation turbulence model is important since the DES uses the same transport equation for the turbulence stresses closure. The RANS simulation with the Spalart-Allmaras one equation turbulence model is the foundation for DES and is hence validated with other transonic flows including a 2D subsonic flat plate turbulent boundary layer, 2D transonic inlet-diffuser, 2D RAE2822 airfoil, 3D ONERA M6 wing, and a 3D transonic duct with shock boundary layer interaction. The predicted results agree very well with the experiments. The RANS code is then further used to study the slot size effect of a co-flow jet (CFJ) airfoil. The DES solver with fully coupled fluid-structural interaction methodology is validated with vortex induced vibration of a cylinder and a transonic forced pitching airfoil. For the cylinder, the laminar Navier-Stokes equations are solved due to the low Reynolds number. The 3D effects are observed in both stationary and oscillating cylinder simulation because of the flow separations behind the cylinder. For the transonic forced pitching airfoil DES computation, there is no flow separation in the flow field. The DES results agree well with the RANS results. These two cases indicate that the DES is more effective on predicting flow separation. The DES code is used to simulate the limited cycle oscillation of NLR7301 airfoil. For the cases computed in this research, the predicted LCO frequency, amplitudes, averaged lift and moment, all agree excellently with the experiment. The solutions appear to have bifurcation and are dependent on the initial perturbation. The developed methodology is able to capture the LCO with very small amplitudes measured in the experiment. This is attributed to the high order low diffusion schemes, fully coupled FSI model, and the turbulence model used. This research appears to be the first time that a numerical simulation of LCO matches the experiment. The DES code is also used to simulate the CFJ airfoil jet mixing at high angle of attack. In conclusion, the numerical strategy of the high order DES with fully coupled FSI model and parallel computing developed in this research is demonstrated to have high accuracy, robustness, and efficiency. Future work to further maturate the methodology is suggested.

Turbocharging the internal combustion (IC) engine is a common technique to increase the power density. If turbocharging is used with the downsizing technique, the fuel consumption and pollution of green house gases can be decreased. In the turbocharger, the energy of the engine exhaust gas is extracted by expanding it through the turbine which drives the compressor by a shaft. If a turbocharged IC engine is compared with a natural aspirated engine, the turbocharged engine will be smaller, lighter and will also have a better efficiency, due to less pump losses, lower inertia of the system and less friction losses. To be able to further increase the efficiency of the IC engine, the understanding of the highly unsteady flow in turbochargers must be improved, which then can be used to increase the efficiency of the turbine and the compressor. The main objective with this thesis has been to enhance the understanding of the unsteady flow in turbocharger and to assess the sensitivity of inflow conditions on the turbocharger performance. The performance and the flow field in a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has been assessed by using Large Eddy Simulation (LES). To assess the effects of different operation conditions on the turbine performance, different cases have been considered with different perturbations and unsteadiness of the inflow conditions. Also different rotational speeds of the turbine wheel were considered. The results show that the turbine cannot be treated as being quasi-stationary; for example,the shaft power varies for different frequencies of the pulses for the same amplitude of mass flow. The results also show that perturbations and unsteadiness that are created in the geometry upstream of the turbine have substantial effects on the performance of the turbocharger. All this can be summarized as that perturbations and unsteadiness in the inflow conditions to the turbine affect the performance. The unsteady flow field in ported shroud compressor has also been assessed by using LES for two different operational points. For an operational point near surge, the flow field in the entire compressor stage is unsteady, where the driving mechanism is an unsteadiness created in the volute. For an operational point far away from surge, the flow field in the compressor is relatively much more steady as compared with the former case. Although the stable operational point exhibits back-flow from the ported shroud channels, which implies that the flow into the compressor wheel is disturbed due to the structures that are created in the shear layer between the bulk flow and the back-flow from the ported shroud channels. / QC20100622

Buoyant flows are characterized with unsteady large-scale structures and thus time-dependent large eddy simulation (LES) is generally favored. In this dissertation, to further explore LES for buoyant flow, an LES code based on a collocated grid system is first developed. A multigrid solver using a control strategy is developed for the pressure Poisson equations. The control strategy significantly accelerated the convergence rate. A temperature solver using a fourth-order Runge-Kutta approach is also developed. The LES code is extensively tested before it is applied. Although the collocated grid system will introduce conservation errors, in tests of a steady lid-driven cavity flow and transient start-up flow, the effect of the non-conservation of the collocated grid system was not significant. <p>In LES, the effect of SGS scales is represented by SGS models. A novel dynamic nonlinear model (DNM) for SGS stress is tested using isothermal channel flow at Reynolds number 395. The kinetic energy dissipation and geometrical characteristics of the resolved scale and SGS scale with respect to the DNM are investigated. In general, the DNM is reliable and has relatively realistic geometrical properties in comparison with the conventional dynamic model in the present study. In contrast to a pure advecting velocity field, a scalar (temperature) field displays very different characteristics. The modelling of SGS heat flux has not been as extensively studied as that of SGS stress partly due to the complexity of the scalar transport. In this dissertation, LES for a turbulent combined forced and natural convection is studied. The DNM model and a nonlinear dynamic tensor diffusivity model (DTDM-HF) are applied for the SGS stress and heat flux, respectively. The combined effect of the nonlinear models is compared to that of linear models. Notable differences between the nonlinear and linear SGS models are observed at the subgrid-scale level. At the resolved scale, the difference is smaller but relatively more distinguishable in terms of quantities related to the temperature field. <p>Finally, the geometrical properties of the resolved velocity and temperature fields of the thermal flow are investigated based on the LES prediction. Some universal geometrical patterns have been reproduced, e.g. the positively skewed resolved enstrophy generation and the alignment between the vorticity and vortex stretching vectors. The present research demonstrates that LES is an effective tool for the study of the geometrical properties of a turbulent flow at the resolved-scales. The wall imposed anisotropy on the flow structures and orientation of the SGS heat flux vector are also specifically examined. In contrast to the dynamic eddy diffusivity model, the DTDM-HF successfully predicts the near-wall physics and demonstrates a non-alignment pattern between the SGS heat flux and temperature gradient vector.

Large-eddy simulation (LES) is emerging as a promising computational tool for reacting flows. High-order schemes for LES are desirable to achieve improved solution accuracy with reduced computational cost. In this study, a parallel, block-based, three-dimensional high-order central essentially non-oscillatory (CENO) finite-volume scheme for LES of premixed turbulent combustion is developed for Cartesian mesh. This LES formulation makes use of the flame surface density (FSD) for subfilter-scale reaction rate modelling. An algebraic model is used to approximate the FSD. A detailed explanation of the governing equations for LES and the mathematical framework for CENO schemes are presented. The CENO reconstruction is validated and is also applied to three-dimensional Euler equations prior to its application to the equations governing LES of reacting flows.